BOSTON (Reuters) - Massachusetts' top court on Tuesday weighed whether it should declare a requirement that people must register to vote 20 days before an election unconstitutional, in a case that could impact the ability of thousands of citizens to cast ballots.

The Massachusetts Supreme Judicial Court heard arguments in an appeal by the state's top election official of a ruling by a lower-court judge in July holding that the registration cutoff violates the state's constitution.

Secretary of the Commonwealth William Galvin, a Democrat who oversees the state's elections, appealed the ruling, arguing that the 20-day rule did not impose a severe burden on voting rights.

He has proposed that the legislature pass a new law allowing election day registration. Assistant State Solicitor David Kravitz, representing Galvin, told the court that the issue of how to regulate registration should be left to the legislature to decide.

"The legislature may and in fact must put in place some sort of system to ascertain who is qualified and who is not," he argued in court.

But Chief Justice Ralph Gants questioned how Kravitz could argue the 20-day rule did not impose a severe burden on voting rights, citing figures suggesting that some people had been prevented from casting ballots due to the law.

"Is the denial of their right to vote not a severe burden?" Gants asked.

The arguments came in a lawsuit filed in 2016 by the American Civil Liberties Union on behalf of two organizations, Chelsea Collaborative and MassVote, and several individual qualified voters.

The ACLU contends Massachusetts' restriction disenfranchises thousands of otherwise qualified voters. According to a brief the state filed, 34 states have deadlines requiring voters to register seven to 30 days before elections.

Suffolk County Superior Court Judge Douglas Wilkins in July ruled that while Massachusetts could pass laws to ensure voter qualifications and election security, evidence presented at a trial showed no such necessity for the registration cutoff.

Jessie Rossman, a lawyer with the American Civil Liberties Union of Massachusetts, argued that the 20-day requirement was "nowhere near" what state officials and city clerks needed to prepare for an election.

She noted that in 2016, Massachusetts began allowing voters to vote early before an election just five days after the 20-day registration cutoff.

"There is no evidence this 20 day deadline is necessary, warranted, or even rational," Rossman said.